The present invention relates to microelectromechanical system (MEMS) valve structures, and more particularly to low-power, high speed electrostatically actuating MEMS valve structures and the associated fabrication methods.
Advances in thin film technology have enabled the development of sophisticated integrated circuits. This advanced semiconductor technology has also been leveraged to create MEMS (Micro Electro Mechanical System) structures. MEMS structures are typically capable of motion or applying force. Many different varieties of MEMS devices have been created, including microsensors, microgears, micromotors, and other microengineered devices. MEMS devices are being developed for a wide variety of applications because they provide the advantages of low cost, high reliability and extremely small size.
Design freedom afforded to engineers of MEMS devices has led to the development of various techniques and structures for providing the force necessary to cause the desired motion within microstructures. For example, microcantilevers have been used to apply rotational mechanical force to rotate micromachined springs and gears. Electromagnetic fields have been used to drive micromotors. Piezoelectric forces have also been successfully been used to controllably move micromachined structures. Controlled thermal expansion of actuators or other MEMS components has been used to create forces for driving microdevices. One such device is found in U.S. Pat. No. 5,475,318 entitled xe2x80x9cMicroprobexe2x80x9d issued Dec. 12, 1995 in the name of inventors Marcus et al., which leverages thermal expansion to move a microdevice. A micro cantilever is constructed from materials having different thermal coefficients of expansion. When heated, the bimorph layers arch differently, causing the micro cantilever to move accordingly. A similar mechanism is used to activate a micromachined thermal switch as described in U.S. Pat. No. 5,463,233 entitled xe2x80x9cMicromachined Thermal Switchxe2x80x9d issued Oct. 31, 1995 in the name of inventor Norling.
Electrostatic forces have also been used to move structures. Traditional electrostatic devices were constructed from laminated films cut from plastic or mylar materials. A flexible electrode was attached to the film, and another electrode was affixed to a base structure. Electrically energizing the respective electrodes created an electrostatic force attracting the electrodes to each other or repelling them from each other. A representative example of these devices is found in U.S. Pat. No. 4,266,339 entitled xe2x80x9cMethod for Making Rolling Electrode for Electrostatic Devicexe2x80x9d issued May 12, 1981 in the name of inventor Kalt. These devices work well for typical motive applications, but these devices cannot be constructed in dimensions suitable for miniaturized integrated circuits, biomedical applications, or MEMS structures.
MEMS electrostatic devices are used advantageously in various applications because of their extremely small size. Electrostatic forces due to the electric field between electrical charges can generate relatively large forces given the small electrode separations inherent in MEMS devices. An example of these devices can be found in U.S. patent application Ser. No. 09/345,300 entitled xe2x80x9cARC resistant High Voltage Micromachined Electrostatic Switchxe2x80x9d filed on Jun. 30, 1999 in the name of inventor Goodwin-Johansson and U.S. patent application Ser. No. 09/320,891 entitled xe2x80x9cMicromachined Electrostatic Actuator with Air Gapxe2x80x9d filed on May 27, 1999 in the name of inventor Goodwin-Johansson. Both of these applications are assigned to MCNC, the assignee of the present invention.
Typical MEMS valves have employed thermal actuation/activation methods to control valves with high flow rates (i.e. large apertures and large clearance areas around the aperture). Thermal actuation has been preferred because it is able to provide the large forces necessary to control the valve over the requisite large distances. However, these valves have relatively slow operation rates due to the thermal time constraints related to the valve materials. Additionally, thermally activated MEMS valves use resistive heating where the power consumed is calculated by the current squared times the resistance and considerable power is consumed in operating the valve.
It would be advantageous to construct a MEMS valve device using electrostatic actuation that is capable of both large displacements and large forces. The electrostatic nature of the MEMS valve will allow for relatively low power consumption and, therefore, no unwarranted heating of the flowing gas or fluid would occur. Additionally, the electrostatic valve will provide for relatively fast operation, allowing for more precise control of the open and closed states of the valve. In addition, it would be advantageous to develop a MEMS valve that forms a secure valve seat to valve cover interface to assure low leakage rates are realized. It would also be beneficial to provide for a MEMS valve that minimizes the occurrence of stiction between the substrate and moveable membrane. Stiction, which is a well-known concept in microelectronics, is defined as the tendency for contacting MEMS surfaces to stick to one another. Stiction is especially a concern in valve devices in which a pressure differential exists across the closed valve. It would be beneficial to devise a MEMS valve that relieves the pressure differential prior to opening the valve.
As such, MEMS electrostatic valves that have improved performance characteristics are desired for many applications. For example, micromachined valves capable of fast actuation, large valve force and large valve flap displacements that utilize minimal power are desirable, but are currently unavailable.
The present invention provides for improved MEMS electrostatic valves that benefit from large valve force, fast actuation and large displacement of the moveable membrane to allow for the efficient transport of increased amounts of gas or liquid through the valve. Further, methods for making the MEMS electrostatic valve according to the present invention are provided.
A MEMS valve device driven by electrostatic forces according to the present invention comprises a planar substrate having an aperture formed therein and substrate electrode disposed on the planar substrate. Further, the MEMS valve device of the present invention includes a moveable membrane that overlies the aperture and has an electrode element and a biasing element. The moveable membrane is defined horizontally as having a fixed portion attached to the substrate and a distal portion that is moveable with respect the substrate. Additionally, at least one resiliently compressible dielectric layer is provided to insure electrical isolation between the substrate electrode and electrode element of the moveable membrane. In operation, a voltage differential is established between the substrate electrode and the electrode element of the moveable membrane to move the membrane relative to the aperture to thereby controllably adjust the portion of the aperture that is covered by the membrane.
In one embodiment of the MEMS valve device according to the present invention the resiliently compressible dielectric layer is formed on the substrate electrode and provides for the valve seat surface. In another embodiment of the present invention the resiliently compressible dielectric layer is formed on the moveable membrane and provides for the valve seal surface. In yet another embodiment resiliently compressible dielectric layers are formed on both the substrate electrode and the moveable membrane and provide for both the valve seat surface and the valve seal surface. The resiliently compressible nature of the dielectric layer allows for a secure closed valve to form that benefits from a low leakage rate.
In yet another embodiment the resiliently compressible dielectric layer has a textured surface; either at the valve seat, the valve seal or at both surfaces. By texturing these surfaces the valve is able to overcome stiction that causes the MEMS films to stick together after the electrostatic voltage is removed. In effect, texturing reduces the surface area around the valve seat to seal interface thereby reducing the effects of stiction. Additionally, texturing allows pressure to be advantageously used in easing the opening of the valve.
In another embodiment of the invention a pressure-relieving aperture is defined within the planar substrate and is positioned to underlie the moveable membrane. The pressure-relieving aperture provides a decrease in the pressure differential across the valve aperture by alleviating pressure prior to the opening of the valve.
Alternatively, another embodiment of the present invention provides for a MEMS valve array driven by electrostatic forces. The MEMS valve array comprises a planar substrate having a plurality of apertures defined therein and a substrate signal electrode disposed on the planar substrate. Further, the MEMS valve device of the present invention includes a moveable membrane that overlies the plurality of apertures and has an electrode element and a biasing element. The moveable membrane is defined horizontally as having a fixed portion attached to the substrate and a distal portion that is moveable with respect the substrate. Additionally, at least one resiliently compressible dielectric layer is provided to insure electrical isolation between the substrate electrode and electrode element of the moveable membrane. The array configuration allows for increased gas or liquid flows.
In one embodiment of the array, the substrate has a plurality of apertures and a plurality of moveable membranes is provided; wherein each aperture has a corresponding moveable membrane. In this manner, the electrode elements of the moveable membranes can be individually supplied electrostatic voltages, thus controlling the number of apertures opened or closed. This configuration effectuates a variable flow rate valve.
In another embodiment of the array, the substrate has a plurality of apertures and a plurality of substrate electrodes is provided; wherein each aperture has a corresponding substrate electrode. In this manner, the substrate electrodes can be individually supplied electrostatic voltages, thus controlling the number of apertures opened or closed. This configuration effectuates a variable flow rate valve.
Additionally, the array of the present invention is embodied in a substrate having a plurality of apertures and a shaped electrode element within the moveable membrane and/or the substrate. The shaped nature of the electrode element allows for the amount of membrane uncurling to be adjusted in accordance with the amount of voltage applied between the electrodes.
Alternately, another embodiment of the present invention provides a method for making the MEMS valve device described above. The method comprises the steps of etching the frontside of a substrate to define an aperture extending partially through the substrate, filling the aperture with a plug material, forming a membrane valve structure on the frontside of the substrate, removing the plug material, etching the backside of the valve aperture up to the release layer and removing the release layer to at least partially release the membrane from the substrate. The method provided allows for the alignment of the aperture and the substrate electrode to be accomplished on the frontside of the substrate.
As such the MEMS valve device driven by electrostatic force is capable of both large displacements and large forces. The electrostatic nature of the MEMS valve allows for relatively low power consumption and, therefore, no unwarranted heating of the flowing gas or fluid occurs. Additionally, the electrostatic valve will provide for relatively fast operation, allowing for faster cycle time and more precise control of the open and closed states. Furthermore, the MEMS valve provides for a secure valve seat to valve cover interface to assure low leakage rates. An additional benefit is realized in providing for a MEMS valve that minimizes the occurrence of stiction between the substrate and moveable membrane. Stiction is overcome by providing for textured surfaces at the valve seat and/or valve seal interface or allowing for a pressure-relieving aperture to be defined in the substrate. These and many more advantages can be realized with the MEMS valve device of the present invention.